This is way out of left field, but if anyone is interested in fiction / deep fantasy:
Malazan Book of the Fallen series
10 books in the series, totaling ~11,200 pages @ a whopping 3.3M word count. For reference, the Harry Potter series total word count is just over 1M.
Honestly, it puts Lord of the Rings & Game of Thrones to shame imo. The world is so mind-blowingly expansive and fleshed out, it makes you weep when you finish a book because you just want it to go on forever. Thankfully though there are 10 books, averaging 1,100 pages per book. It’s incredible, can’t recommend the series enough for fantasy lovers!